Coatings on aluminum



Patented May 24, 1938 UNITED STATES COATINGS N ALUMINUM James W.Newsome, New Kensington, Pa., as-

signor to Aluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Application June 22, 1937,

Serial No. 149,680

8 Claims.

This invention relates to the production of oxide-coated aluminumsurfaces, and it is particularly concerned with a method for theproduction of a substantially colorless, hard, adherent and adsorptivecoating consisting substantial, ly of aluminum oxide on an aluminumsurface by chemical treatment without the external application ofelectric current. as used herein and in the appended claims is in- 10tended to include both aluminum in its various degrees of purity and thealuminum base alloys.

Oxide coatings have found extensive application for the protection anddecoration of aluminum surfaces. This field has been extendedparticularly in recent years as a result of the development of theso-called anodic oxidation methods in .which hard, adherent,abrasion-resistant coatings are formed on aluminum-by electrolyticoxidation. These coatings are generally more or less porous andadsorbent, and can be colored, impregnated with corrosion-inhibiting 51b stances, sealed, or otherwise treated, as their particular usedemands. However, thesemethods do not lend themselves readily to theproduction of oxide coatings on small articles, such as buttons,eyelets, and the like, and their use is not eco-' nomical in theproduction of certain types of articles intended for sale at relative ylow prices.

While it has been recognized that the methods of producing coatings onaluminum by simple chemical treatment without-the application ofelectrical energywould be desirable for use in the coating of smallarticles, the methods heretofore available, and known to the art, forproviding oxide coatings on aluminum by a simple chemical treatment havenot produced coatings which have a suitable combination of properties torecommend them for general use. In order to be satisfactory for generaluse, it is desirable that 40 the coating produced should be sufficientlyhard and adherent to resist the abrasion incident to ordinary use, andshould preferably be substantially colorless and more or less adsorbent,so that it can be colored by dyeing or pigmenting. It is an object ofthis invention-to provide a chemical treatment method for the productionof an oxide coating on aluminum having a combination of propertiespermitting its general use as a finishing coating. It is moreparticularly an object of this invention to provide a method for theproduction of a hard, adherent, adsorbent and substantially colorlesscoating comprising aluminum oxide on an aluminum surface by simplechemical treatment without the aid of an electric current.

The term aluminum It is a further object of this invention to provide acoating of the above nature and properties which is resistant to.abrasion and corrosion, and which is free from pits and discolorationsfre quently encountered'with former chemical coat- 5 ings applied by theusual chemical treatments.

It is a still further object of the invention to provide a chemicalcoating bath which may be used continuously over a relatively extendedperiod of time while continuing to maintain equilibrium 10 conditions ofreagent concentration, so that it functions uniformly over the term ofits use.

I have discovered that improved coatings of the type describedhereinabove can be produced on aluminum and aluminum alloy surfaces by15 treatment of the said surfaces with a solution containing an. alkalimetal salt of an organic hydroxy acid and an alkali metal aluminate. Thetreatment is preferably carried out in a sodium aluminate solutioncontaining a substance of the 20 group composed of sodium glycollate,-sodium lactate, sodium tartrat'e, sodium citrate, sodium malate, sodiumsalicylate, and sodium para hydroxy benzoate. The article to be coatedis preferably immersed in the solution at an elevated 25 temperature.The reaction between the coating solution and the metal surface, whichis at first fairly vigorous, gradually becomes slower and finallypractically stops, generally within twenty to thirty minutes whenoperated at temperatures 30 of from to centigracle. The coating producedis relatively hard, adherent, abrasion-resistant andcorrosion-resistant, and is relatively free from pits and streaks. It ismore or less adsorbent and substantially colorless. 35

Satisfactory solutions may be obtained by using aluminate solutionscontaining as littleas 0.l per cent of sodium aluminate or as much as2.0 per cent, but forzmost purposes a concentration of L 0.5 to.1.0 percent has proved most satisfactory. 4,0

The amount of the salt of the organic hydroxy acid may vary somewhatdepending on the particular salt used. In general, when the sodium saltsare used, amounts ranging from about 0.025 per cent to 1.5 per cent haveproved satisfactory. 45 With the sodium salts of glycollic, lactic,tartaric, citric, malic, and para hydroxy benzoic acids, I have found itpreferable to use asolution in which the ratio of the sodium salt of theorganic hydroxy acidto the sodium aluminate is 50 from about 1:1 to 2:1,and I prefer in most cases to use a solution containing 0.5 to 1.0 percent of the salt of the organic hydroxy acid and 0.5 per cent of sodiumaluminate. The sodium salicylate seems to be more effective, and it isdesirable to 65 or in a ratio of about one part salicylate to 2 to 4parts aluminate. It is preferred first. to make up an aqueous solutionof the salt of the organic hydroxy acid and then add the alkali metalaluminate. When the aluminum or aluminum alloy article is immersed inthe solution, preferably between 85 and 95 C. as aforesaid, the solutionrapidly attains equilibrium conditions, and any excess of alumina causedby the action of the bath in producing the oxide coating precipitatesout and settles at the bottom of the container.

Preliminary cleaning of the aluminum surface to be coated is notessential, thiiugh it maybe desirable. I have found that aluminumsurfaces on which there is a film of oil may be satisfactorily coatedwhen treated according to my invention, without any preliminarytreatment to remove the grease. however, to use a clean aluminumsurface. In some cases it may be preferred to preliminarily etch thesurface to be coated. Particularly good The properties" of the coatingobtained by the method of my invention will vary somewhat with thecomposition of the aluminum or aluminum It is generally preferred,

alloy surface treated, and the exact conditions of treatment used.Certain aluminum alloys, by reason ofthe presence of elements such asiron, manganese, or copper, have a tendency to develop coloration in thecoatings produced, and in such cases the use of somewhat weakersolutions than those noted in the preferred solutions above referred tomay be desirable. Although the preferred temperature range has beenstated as between 85 and 95 C., it may vary from room temperature td theboiling point. The time required to produce a coating of the requiredthickness at the lower temperature must, of course, be lengthened insuch instance, and at room temperature frqm 10 to 20 hours may berequired.

As a specific example of the method of coating aluminum according to theprocess of my inven-' tion, I may cite the following. The aluminum to becoated was immersed in a solution containing 1.0 per cent sodiumaluminate and 1.0 .per cent sodium tartrate. at a temperature of 85 C.,and the treatment continued for a period of about 30 minutes. Aftertreatment the aluminum was found to be coated with a hard, adherentcoating having an average thickness, as determined by microscopicexamination of a cross section, of about 0.00008 inch. This coating wassubstantially colorless, and was hard, abrasion-resistant and of goodcorrosion resistance.

I claim:

1. A method ofproducing a substantially col- The solution was maintainedorless protective coating on an aluminum surface, comprising treatingsaid surface with a solution containing an alkali metal salt of anorganic hydroxy acid andan alkali metal aluminate.

2. A method of producing a substantially colorless protective coating onan: aluminum surface, comprising treating said surface with a .solutioncontaining an alkali metal salt of an organic hydroxy'acid and an alkalimetal aluminate at a temperature of 85 to 95 centigrade. 3. A method ofproducing a substantially colorless protective coating on an aluminumsurface, comprising treating said surface with a solution containing analkali metal aluminate and 0.025 to 1.5 per cent .of a sodium salt of anorganic hydroxy acid selected from the group consisting of glycollicacid, lactic acid, tartaric acid, citric acid, malic acid, salicylicacid, and para hydroxy benzoic acid.

4. A method of producing a substantially colorless protective coating onan aluminum surface, comprising treating said surface with a solutioncontaining from 0.1 to 2.0 per cent of sodium aluminate and a sodiumsalt of an organic hydroxy acid selected from the group consisting ofglycollic acid, lactic acid, tartaric acid, citric acid, malic acid,salicylic acid and para hydroxy benzoic' acid.

5. A method of producing a substantially colorless protective coating onan aluminum surface, comprising treating said surface with a solutioncontaining from 0.1 to 2.0 per cent of sodium aluminate and a sodiumsalt of an organic hydroxy acid selected from the group consisting ofglycollic acid, lactic acid, tartaric acid, citric acid, malic acid, andpara hydroxy benzoic acid in which the ratio of the sodium salt of theorganic hydroxy acid to the sodium aluminate is from about 1:1 to'2:1.

6. A method of producing a substantially colorless protective coating onan aluminum surface, comprising treating said surface with a solutioncontaining about 0.5 per cent of sodium aluminate and m 1.0 per cent ofa salt of an organic hydroxy acid selected from the group of hydroxyacids consisting of glycollic acid, lactic acid, tartaric acid, citricacid, malic acid, and para hydroxy benzoic acid.

- 7. A method of producing a substantially colorless protective coatingon an aluminum surface, comprising treating said surface with a solutioncontaining sodium aluminate and sodium salicylate in a ratio of aboutone part of salicylate to 2 to 4 parts aluminate. 8. A method ofproducing a substantially colorless protective coating on an' aluminumsurface,

' comp ing treating said surface with a solution cont ning about 0.25per cent of sodium salicylate and 0.5 to 1.0 per cent of sodiumaluminate. JAMES W. NEWSOME.

